1. Field of the Invention
The present invention relates to a porous film and a method for manufacturing the same and, more particularly, to porous films useful as various functional films such as a gas permeability control film and a water vapor permeability control film, and a method of manufacturing the same.
2. Description of the Related Art
Conventional various porous films serving as various functional films have been developed.
For example, a fine inorganic powder is filled in a general-purpose olefin resin (e.g., polyethylene) in a large amount (generally, 50 vol % or more) and is formed into a film. The film is then uniaxially or biaxially stretched at a high ratio. According to this method, open cells are formed in the interface between the resin and the inorganic powder to form a large number of pores which communicate with each other in the form of a maze, thereby manufacturing a porous polyethylene film. However, since a large amount of inorganic powder is added in the porous film, the essential properties (e.g., strength, softness, and transparency) of a resin constituting a film are considerably deteriorated.
It is known to manufacture a porous thermoplastic resin film by a mechanical perforation method such as a needle punch method or a heat fusion perforation method. In the needle punch method, a thermoplastic resin film (e.g., a polyethylene film) is perforated by pressing heated needles against the film. In the heat fusion perforation method, a thermoplastic resin (e.g., a polyethylene film) is perforated by fusing the film by using a heated embossing roll.
A large number of through pores are formed in the porous thermoplastic resin film formed by a mechanical perforation method. For this reason, the size and number of through pores formed in the porous thermoplastic resin film are adjusted to control the gas permeation amount (e.g., the oxygen gas permeation amount). However, water, bacteria, and viruses also permeate through the through pores of the porous thermoplastic resin film. In addition, since the size of each through pore is as large as about 100 .mu.m, it is difficult to highly accurately control the oxygen gas permeation amount.